Small change in Maps API reference

[VictorVelarde]

• url: https://carto.com/developers/maps-api/reference/#operation/instantiateAnonymousMap

It looks like spec is not updated, or at least we saw a small change regarding to last_updated field, currently updated_at. That might also come from a version 1.0.0 vs 1.3.1 change.

/cc @dgaubert
@CartoDB/dc-managers